1. Field of the Invention
The present invention relates to a color display tube device used in a monitor, a television receiver or the like.
2. Description of the Related Art
One of the major image quality factors of a color display tube is a raster distortion. Conventionally, an upper and lower pincushion distortion and a right and left pincushion distortion in edge portions are corrected by a magnetic field of a deflection yoke and a correction circuit of the color display tube device. However, even when these distortions at the top, bottom, right and left edges are optimized, pincushion distortions sometimes remain in the middle of vertical lines or horizontal lines. They are called xe2x80x9ca right and left inner pincushion distortionxe2x80x9d (see FIG. 6) and xe2x80x9can upper and lower inner pincushion distortionxe2x80x9d (see FIG. 7) respectively. xe2x80x9cThe right and left inner pincushion distortionxe2x80x9d also is called xe2x80x9ca vertical-line inner pincushion distortion,xe2x80x9d and xe2x80x9cthe upper and lower inner pincushion distortionxe2x80x9d also is called xe2x80x9ca horizontal-line inner pincushion distortion.xe2x80x9d
In general, when a displacement amount of a position of a luminescent spot on a flat screen is expressed by X(t) and a deflection angle is expressed by xcex8, X(t) is proportional to tanxcex8. Therefore, a displacement amount in a horizontal direction increases gradually toward the edge portions in a horizontal direction of a phosphor screen, so as to cause a non-linear distortion, called xe2x80x9cS-shaped distortion.xe2x80x9d This S-shaped distortion is corrected by providing an S-shaped distortion correction circuit. A necessary correction amount of the S-shaped distortion is inversely proportional to a vertical distance from a deflection center to each spot on the phosphor screen. As shown in FIG. 8, since the upper and lower portions of the raster of the color display tube, that is, edge portions 18 of the phosphor screen in the vertical direction is farther from the deflection center than a middle portion 17 (a hatched area), the necessary correction amount of the S-shaped distortion is smaller in the edge portions 18 than in the middle portion 17.
However, although the S-shaped distortion correction circuit of the conventional color display tube device optimizes the S-shaped distortion correction in the middle portion of the phosphor screen in the vertical direction, there is a problem as follows. Because the difference of the necessary correction amount between the middle portion and the edge portions of the phosphor screen in the vertical direction was not considered, the S-shaped distortion was corrected too much in the edge portions, thus causing the right and left inner pincushion distortion. In particular, accompanying the development of flatter front panels and larger deflection angles in the color display tubes, this right and left inner pincushion distortion was becoming a noticeable problem.
In response to such a problem, the invention described in JP 9-149283 A had a configuration that a horizontal deflection current is passed through a saturable reactor and modulated by a vertical deflection current, so that during a deflection toward the upper and lower portions of the raster, i.e., the edge portions in the vertical direction, a total inductance of a horizontal deflection portion becomes smaller in deflecting an electron beam toward the edge portions in the horizontal direction (i.e., in deflecting an electron beam toward the diagonal portions of the phosphor screen), thereby removing the right and left inner pincushion distortion.
A trend toward flatter front panels and larger deflection angles in the color display tubes in recent years has caused a difference of the S-shaped distortion correction amount between the left and right portions and the middle portion in the horizontal direction of the raster, as well as the vertical direction of the raster. As a result, not only the right and left inner pincushion distortion, but also the upper and lower inner pincushion distortion has become a problem. The conventional inner pincushion distortion correction circuit described in the above-mentioned JP 9-149283 A was effective in correcting the right and left inner pincushion distortion, but could not correct the upper and lower inner pincushion distortion. Thus, the distortion conventionally was balanced vertically by tilting a central axis of the deflection yoke in the vertical direction, which was called a vertical tilt.
Alternatively, in JP 6-283115 A and JP 63-80756 U, an upper and lower barrel distortion in the middle portion in the vertical direction was corrected by arranging magnets at four corners of the deflection yoke. However, this system had a problem in that a poor horizontal line convergence of red and blue beams in the middle portion in the vertical direction showed up when correcting the raster distortion, and the installation of the magnets increased manufacturing steps so as to lower a work efficiency.
It is an object of the present invention to provide a color display tube device with a system for correcting an upper and lower inner pincushion distortion while correcting an upper and lower pincushion distortion, and for achieving a material cost reduction, a reduction of manufacturing steps and a higher work efficiency because it is sufficient that two magnets each are installed on the upper and lower sides.
A color display tube device of the present invention includes a glass bulb having a phosphor screen on an inner surface thereof, an in-line electron gun that is arranged in the glass bulb and irradiates an electron beam on the phosphor screen, and a deflection device that is arranged outside the glass bulb and has a horizontal deflection coil and a vertical deflection coil. A first magnetic field generator for generating a magnetic field having a same polarity as a magnetic field generated by the vertical deflection coil during a deflection toward an upper side is provided above a horizontal axis of the deflection device, and a second magnetic field generator for generating a magnetic field having a same polarity as a magnetic field generated by the vertical deflection coil during a deflection toward a lower side is provided below the horizontal axis of the deflection device. A third magnetic field generator for generating a magnetic field having an opposite polarity to the magnetic field generated by the vertical deflection coil during the deflection toward the upper side is provided above the horizontal axis of the deflection device, and a fourth magnetic field generator for generating a magnetic field having an opposite polarity to the magnetic field generated by the vertical deflection coil during the deflection toward the lower side is provided below the horizontal axis of the deflection device. The first and second magnetic field generators are arranged on a side of the phosphor screen with respect to a peak position of a deflection magnetic field strength of the horizontal deflection coil and the vertical deflection coil in a tube axis direction. The third and fourth magnetic field generators are arranged at a same position as the first and second magnetic field generators or on the side of the phosphor screen with respect to the first and second magnetic field generators in the tube axis direction.
In accordance with this configuration, the magnetic field having the same polarity with that of the vertical deflection coil generated by the first and second magnetic field generators can correct the upper and lower inner pincushion distortion in the middle portion in the vertical direction of the phosphor screen and also over-correct the upper and lower pincushion distortion in the edge portions in the vertical direction, and the magnetic field having the opposite polarity to that of the vertical deflection coil generated by the third and fourth magnetic field generators can recorrect only the distortion in the edge portions in the vertical direction that has been over-corrected, thereby correcting both the upper and lower pincushion distortion in the edge portions and the upper and lower inner pincushion distortion in the middle portion.
In the above configuration, it is preferable that the magnetic field strength of the third and fourth magnetic field generators is smaller than that of the first and second magnetic field generators, and the distance from the third and fourth magnetic field generators to the glass bulb is smaller than that from the first and second magnetic field generators to the glass bulb in a vertical axis direction.
In accordance with this preferable configuration, a magnetic field in an attenuation region of the first and second magnetic field generators can exert substantially equal forces in a correction direction on an electron beam to be deflected toward the edge portions in the vertical direction and an electron beam to be deflected toward the middle portion in the vertical direction, and a magnetic field generated by the third and fourth magnetic field generators can exert a force in a direction opposite to the correction direction only on an electron beam to be deflected toward the edge portions in the vertical direction. Thus, the upper and lower inner pincushion distortion can be corrected very effectively.
Also, in the color display tube device of the present invention, it is preferable that the first to fourth magnetic field generators include magnets.
In accordance with this preferable configuration, the size, shape and magnetic characteristics etc. of the magnets constituting the first and second magnetic field generators and the magnets constituting the third and fourth magnetic field generators are selected suitably, thereby adjusting a correction magnetic field having an effect on the electron beams to be deflected toward the edge portions and the middle portion in the vertical direction of the phosphor screen. Thus, it is possible to correct both the upper and lower pincushion distortion in the edge portions and the upper and lower inner pincushion distortion in the middle portion in a simple and low-cost manner.